Is that a wholly bad thing? Yes, according to many conservationists. But I’ve come to believe that the reality is much more complex, and I thought I would describe a bit of this complexity over my next few posts. First, let’s put this demographic moment in its historic context.

The coming urbanization represents an upheaval of the traditional demographic order, one likely to be permanent barring civilization’s collapse. The best archaeological and genetic evidence suggests Homo sapiens diverged from our nearest ancestor, Homo erectus, some 400,000 years ago to 250,000 years ago. For the first few hundred millennia — 95 percent of the time our species has existed — we got our food from hunting and gathering. Cities were not possible in a hunting and gathering society, for almost everyone had to work to procure food and almost no one had time for anything else, much less the construction of elaborate urban structures.

Around 10,000 years ago, the domestication of plants and animals occurred, an event that has rightly been called a revolution in human affairs. At around the same time, cities first appear in the archaeological record, presumably supported by the agriculture surplus of farmers. The urbanist Lewis Mumford once wrote of cities as a great bargain between city dwellers and farmers. Food went into cities, and out came assurances of security and new knowledge, of both a religious and a technological nature.

Urban dwellers remained a very small elite, a miniscule proportion of total population, until the industrial revolution created a big need for labor in cities and advances in agricultural technology reduced the need for human labor during farming. In the United States, this process began in the 19th century. In the 1860 census, just prior to the Civil War, less than 20 percent of Americans lived in urban areas. One generation later, in 1900, 40 percent of Americans were in urban areas. Today, the vast majority of Americans live in a city: Only 21 percent of Americans live in rural areas, and less than 2 percent of Americans are actually farmers.

Sarah Palin famously claimed to prefer “real America,” with the unstated implication that other parts of the United States are not quite American. There is decidedly a small town feel to “real America,” whether it’s Wasilla, Alaska, or the fictional Bedford Falls portrayed in It’s a Wonderful Life. The irony is that Palin was making her claim at a time when few Americans watching actually lived in a small town. If one excludes bedroom communities of major metropolitan areas from the calculation, only 8.1 percent of Americans live in small towns.

In contrast, 80.3 percent of people live in major metropolitan areas or their suburbs. Many conservationists face a somewhat similar divide. While most of us tend to live in towns (that is where the jobs and the universities and the decision-makers are found), our hearts are really in far more rural and wild areas. It is there that most of the functioning natural systems are left, and many of us (myself included) wish we could see more of these wild places. The trees and parks in our urban home are not quite “real Nature,” but some partial version of it.

The rest of developing world is now becoming urbanized, and will soon catch up with the United States and Europe in this respect. For instance:

Sub-Saharan Africa is now 35.0 percent urban, and by 2050 will be 60.5 percent urban.

Southeast Asia is currently 44.1 percent urban, and will reach 73.3 percent urban by 2050.

Through a combination of population increase (more births than deaths) and migration (from rural to urban), the cities of the world will swell by 3.2 billion people by 2050. As one speaker at the third World Urban Forum put it, humanity is doing the equivalent of building a city the size of Vancouver every week. Another way to look at it: By 2030, more houses will be built than currently exist in all of Europe. Humanity is building the great cities of the future — and quickly, hastily.

The logical demographic endpoint is an urban world, where the vast majority of the humanity lives in cities. While demographers are quite familiar with a mostly urban population from looking at U.S. and E.U. figures, at a global level this is something wholly different than the first 99 percent of our species existence. Apparently, this is also a demographically stable state. In the U.S. and E.U. there is no trend away from urbanization, and only in the most severe political or economic crises is there even an occasional return to the countryside. When historians write about the 18th and 19th century, the process of industrialization is in many ways more important than the politicians and monarchs.

Historians will similarly focus on the 20th and 21st centuries as the age of cities and the dawn of an urban world.

Conservationists have to be prepared: Humanity now lives in a world of the city, by the city, and for the city.